Add ignore-dependencies option

With the previous CRD changes in effect, the merge-check pipeline
became rather large since it needlessly stacked dependent (both
git-dependent and commit-dependent) changes for each change tested.
Since commit-dependencies should not affect whether a change has
a merge conflict, and git-dependencies are already taken care of,
add an option to allow independent pipelines to ignore dependencies.

We will only set this for the merge-check pipeline in OpenStack.

Change-Id: I553446374ac12aa3e3f2e4bbea7ca8fafba42294
diff --git a/tests/fixtures/layouts/good_layout.yaml b/tests/fixtures/layouts/good_layout.yaml
index 4bd5e70..fc2effd 100644
--- a/tests/fixtures/layouts/good_layout.yaml
+++ b/tests/fixtures/layouts/good_layout.yaml
@@ -43,6 +43,17 @@
         verified: -2
         workinprogress: true
 
+  - name: merge-check
+    manager: IndependentPipelineManager
+    source: gerrit
+    ignore-dependencies: true
+    trigger:
+      zuul:
+        - event: project-change-merged
+    merge-failure:
+      gerrit:
+        verified: -1
+
 jobs:
   - name: ^.*-merge$
     failure-message: Unable to merge change